Understanding radiobiologic principles and their application to nursing care is integral to assisting optimal outcomes. Radiotherapy aims to treat cancer through delivering sufficient doses of ionising radiation to a specific area of the body to damage target DNA that eventually results in cell death.8 Ionising radiation causes cell death either directly or indirectly.

Direct damage to the atoms that make up the DNA results in either single or double-strand breaks, faulty cross-linking of chains after breakage, damage or loss of a nitrogenous base, or breakage of the hydrogen bond between the two chains of the DNA molecule causing impaired cellular functioning or cell death.1

Indirect damage is caused by the interaction of ionising radiation with the molecules of the cellular fluid, resulting in toxic changes caused by the creation of unstable free radical ions that impair cellular functioning. While a direct hit causes the most lethal damage, the most common injury occurs as a result of the interaction of radiation and the water molecule.13

Radioactive substances emit both gamma rays and radiation particles until all their atoms are stable in a process called radioactive decay. The activity and rate of decay varies from one radioactive source to another. The most important feature of any type of ionising radiation in determining its application in radiotherapy is its penetrating power. The deeper the tumour within a patient, the higher the penetrating power required of the radiation.14

The basic principles of radiobiology are:1, 13, 15, 16

Reoxygenation: occurs when radiation is delivered in multiple fractions to cells that may be relatively resistant due to hypoxia; cells may become reoxygenated and therefore more radiosensitive.

Redistribution: is defined by cells that survive a dose of radiation due to synchronisation in resistant phases of the division cycle and redistributing into more sensitive phases of the cell cycle during subsequent doses of radiation.

Repair: occurs following sub lethal cellular injury which represents damage to the strands of the DNA and which can be repaired by enzymatic processes.

The four Rs of radiation biology, the tumouricidal dose, and the tolerance of surrounding critical tissues determine the prescription for a site specific tumour with a particular histology and pathology.15 To quantify the amount or dose of absorbed radiation within a recipient, the unit of Gray (Gy) is used.

1 Gray (Gy) = 1Joule of energy absorbed per kg of mass = 1J/kg.

This absorbed dose is an indicator of the level of biological effects that may occur in the different tissues of the body due to ionising radiation.14

Based on the principles of radiobiology, the total dose of radiation prescribed to treat a particular tumour is divided into a number of daily doses or fractions. This aims to protect normal surrounding tissue while maximising the radiation effect on the tumour.1 Different tissues have varying tolerance levels to radiation exposure which, if exceeded, results in high morbidity of the treatment.1

Learning activities

Describe the chemical reaction in the cell from the effects of ionising radiation.

Define the term 'half life', and discuss the implications of this concept to delivery of radiotherapy in cancer treatment.

Access a current text. Identify the varying radiation tolerance levels of the following tissues and discuss the impact this has on treatment planning and delivery: